The disclosure relates to a method for preparing precoat on the surface of the filter medium of a polishing filter. The disclosure also relates to polishing filter.
Various filtering methods are well known. One of the most demanding methods is polishing filtration. There the basic principle is to reduce the solids content of process liquids down to an extremely low level, in some applications even down to fractions of a billionth. Polishing filters can be utilized in many different applications, for example in chemical and process industry. Polishing filters can also be utilized in metallurgy plants that use electrolytic concentration processes for removing impurities. Gold and zinc plants are well able to make use of polishing filters.
The purpose of polishing filtration is to separate, as completely as possible, the extremely small solid particles contained in a liquid. The size of the particles varies. Typically the size is smaller than 1,000 nm, and by average within the range 40-400 nm. Generally particles of this size are in English called submicrons, i.e. particles having a size of order of less than one micrometer. In this invention, they are called nanosize particles, which refers to the same thing. Different types of equipment can be used for measuring the size of particles, for example the Hanna Instruments HI 93703 turbidity meter. The device is based on an 825 nm light source, and the measurement is carried out as a reflection of 90 degrees for readings below 40 FTU.
When the solids content of process liquids is reduced down to a very low level, and the particle size remains as small as this, there are several problems in the filtering process. Especially these nanosize particles, i.e. particles with a size of less than one micrometer, tend to block the filter medium in a short time. A particular problem is created by the layer impermeable to liquid that is formed on the surface and pores of the filter medium.
Thus the filter medium, such as a filter cloth or a ceramic medium, is rapidly blocked, which directly affects the filtering efficiency. This in turn increases the filtering costs, such as costs caused directly by the filter material, as well as maintenance and service costs. Moreover, aid medium costs as well as waste and post-processing volumes increase. These are extremely significant aspects considering that in process industry, particularly efficiency and all possible cost benefits are welcome.